A molecular relay inside cells — when a peptide binds a surface receptor, the second messenger carries the signal from the membrane into the interior of the cell.Common second messengers: cAMP (produced by adenylyl cyclase; used by melanocortin, GLP-1, VIP, and secretin receptors), IP3/DAG (produced by phospholipase C; used by Gq-coupled receptors), and Ca²⁺ (calcium ion). Second messengers amplify the original signal — one receptor-ligand binding event can produce thousands of cAMP molecules, which activate thousands of PKA enzyme molecules. This amplification explains why peptide hormones are biologically potent at nanomolar concentrations.